Bolw By ???

The vacuum secondaries open when the motor starts to pull vacuum. When the primaries are wide open the vacumm is zero as the rpm of the motor rises it will get to a point were it tries to draw more air than the two primary butterflies can supply, this starts to create a vaccum. Once the vacuum gets high enough to overcome the secondary diaphragm spring the secondaries start to open. They sell different secondary springs so you can tune at what vacuum you want your secondaries to open.

It is true about the atmospheric pressure at zero vacuum or zero psi but that atmospheric pressure is the same in both the intake and crankcase(as long as there is no crankcase pressure). The pcv port is just that a port, it is not a booster which works with air passing thru it. The only way vapor passes thru the pcv at zero intake vacuum is by positive pressure in the crankcase pushing it's way to the zero pressure environment of the intake manifold. Nothing that runs off of vacuum works at zero intake vacuum, vacuum advance, brake boosters, EGR valves, pcv valves, etc.

Chuck